The reactions of gaseous SO2 with carbon-centered free radicals grafted to a silica surface have been studied by ESR techniques at temperatures above 200 K. Sulfonyl RSO(2)(.) and oxysulfinyl ROSO(.) adducts are reversibly formed in these reactions. The oxysulfinyl adducts appear to be more thermochemically stable; however, the formation of the sulfonyl ones is more kinetically favorable. Vinyl, primary and secondary allyl, and oxymethyl radicals form both types of adducts, while alpha,alpha-dichloroethyl and tertiary alkyl ones yield only oxysulfinyl adducts. ESR characteristics of adducts as well as equilibrium thermodynamic parameters are presented. Low-temperature oxidation of the sulfonyl radicals with dioxygen transforms them into peroxyl radicals, whereas the oxysulfinyl ones are insensitive to oxidation even at ambient temperatures.